2010, Vol.83, No.1

Gas-phase acidities (GA) of ring-substituted (E)-acetophenone oximes, XC6H4C(CH3)=NOH, were determined by measuring proton-transfer equilibria using an FT-ICR mass spectrometer. The magnitude of the substituent effect on the acidity was found to be smaller than that of corresponding phenols by a factor of 0.70. The effects of strong para π-acceptors such as p-NO2 and p-CN are somewhat enhanced compared with those of phenols, indicating that the negative charge on oxygen atom of the conjugate anion of the oxime is significantly delocalized into the aromatic π-system. In addition, it was found that there is a good linear relationship between acidities in gas phase and in DMSO with a slope of 0.26, indicating that the solvation stabilization reduces consistently the effects of substituents without any significant specific solvent effect on a particular substituent. This is in contrast to the phenol acidities in which the effects of electron-rich para +R substituents in DMSO and water were enhanced significantly due to the substituent solvation-assisted resonance (SSAR) effects. These results were consistent with computational studies at the B3LYP/6-311+G(d,p) level of theory.